TOKYO, May 21 (Reuters) - Nippon Life Insurance Co

NPNLI.UL said on Wednesday it has agreed to buy a 20 percent stake in Indonesia's Sequis Life for 4.87 trillion rupiah ($423.9 million), joining its Japanese peers who are expanding in rapidly growing Southeast Asian markets.

PT Asuransi Jiwa Sequis Life, part of Indonesian conglomerate PT Gunung Sewu Kencana, is a middle-sized life insurer with 2.27 trillion rupiah in gross premiums and 9.19 trillion rupiah in assets in 2013.

"The Indonesian life insurance market has been growing robustly against the backdrop of the fourth-largest population in the world of around 250 million and a growing middle class population," Nippon Life said in a statement.

"This high growth is expected to continue in light of the lower life insurance penetration in Indonesia relative to those of developed nations," it added.

Nippon Life, Japan's largest private-sector life insurer, said the deal is subject to approval by Indonesian authorities.

Faced with weak growth prospects amid Japan's ageing population, the country's major life insurers have been aggressively looking for opportunities to expand in Southeast Asian markets.

With Nippon Life's acquisition of the Sequis Life stake, all of Japan's top four private-sector life insurers will have stakes in Indonesian insurers.

In December, Japan's fourth-largest life insurer, Sumitomo Life Insurance Co SMTLI.UL , bought a 40 percent stake in the life insurance unit of PT Bank Negara Indonesia BBNI.JK for 4.2 trillion rupiah ($351.10 million).

Dai-ichi Life Insurance Co 8750.T , the only listed company among Japan's top four life insurers, in October completed the acquisition of a 40 percent stake in Indonesia's Panin Life for $337 million.

($1 = 11487.5 Indonesian rupiah)

(Reporting by Taiga Uranaka; Editing by Matt Driskill)

((taiga.uranaka@thomsonreuters.com)(+81-3-6441-1813)(Reuters Messaging: taiga.uranaka.reuters.com@reuters.net))

Keywords: NIPPON LIFE INS SEQUIS/